Microsoft is going after Google, Box, and Dropbox customers with a new offer for its OneDrive for Business cloud storage service.

If your business is already using cloud storage from one of those three companies, Microsoft will give you OneDrive for free for the duration of your contract with them.

The fine print: Your company needs to have more than 500 users, and you can’t already be a customer of OneDrive or the Office 365 cloud productivity suite.

Google, Box, and Dropbox, be warned: Microsoft is coming for your customers.

On Tuesday, Microsoft announced a new offer to entice companies to use its OneDrive cloud storage service. If you’re a business that’s already using any of those three competitors, Microsoft will give you its OneDrive for Business service for free until your contract with the other company ends.

The promotional offer comes with some caveats: You have to commit to 500 seats or more, meaning small businesses won’t really be able to take advantage. You have to not already be a customer of OneDrive, or of the Microsoft Office 365 productivity suite. And the deal is only good through June 18th.

The big idea is to give customers a taste of what Microsoft can offer, without having to worry about their existing commitments to those other vendors. Microsoft is confident enough that those big customers will sign on, it seems, that they’re willing to swallow the cost. In general, Microsoft is seeing solid growth in its subscription cloud businesses, bolstering its confidence here.

As for why customers would want to make the change: Microsoft is touting the deep integrations between its own OneDrive service and Microsoft Office. While Microsoft Office is compatible with cloud storage from those other companies to a certain degree, there are some special features that Microsoft reserves for its own offerings.

For instance, you need OneDrive if you want to have multiple authors simultaneously work on a single Office document in real-time from the computer or smartphone apps. And OneDrive integrates with other Microsoft apps like the team portal SharePoint, so files stay in sync across the suite.

Plus, Microsoft Global Senior Director Seth Patton hypes up the security of OneDrive, saying that it means most customers’ most stringent requirements.

“That’s a pretty big deal,” Patton tells Business Insider.

Still, with this offer, Microsoft might find itself preaching to the choir somewhat. While Box, Dropbox, and Google’s G Suite certainly have a certain number of large-enterprise customers, they’re mostly known for appealing to smaller and midsize businesses.

Microsoft is already generally considered to be the leader in large companies, given the fact that its Office suite is, and remains, the gold standard in the productivity market. Still, with analysts urging Dropbox to pursue an enterprise-focused strategy ahead of its purported IPO, perhaps Microsoft is just trying to consolidate its lead ahead of time.

After this offer expires, customer will have to pay for OneDrive for Business like usual. Plans start at $US5 per user per month, or else you could opt in for a $US12.50 per user per month subscription to the Office 365 cloud productivity suite – at which point you’d get access to all of the Office applications, like Word, Excel, and PowerPoint, too.